Car overheating, not leaking coolant

I have a very nice Volvo. I have an overheating issue that I cannot seem to figure out. I'm baffled. I could use someone who knows more than me. I could be way off on my reasoning.

Issue:
1. Only in stop-and--go or standstill traffic (or at idle), my car is running hot or overheating. It varies on how long it takes. It does not happen when the car is steadily moving (even at low speeds).

Symptoms:
1. The indicator will say to reduce the speed as the car is running hot.
2. I am not typically losing coolant.
3. When I start to drive, the warning goes away.
4. On a few occasions, it has said that the engine is hot and to stop the engine.
5. When it first started, I could not find a leak and added a bottle of stop leak. Dumb, I know. No lectures. But, it stopped overheating for about a week.
6. When the stop leak did not solve it, I drained but did not flush the system. (The coolant looked new.)
7. With the fresh coolant, I added an additive that also contained stop leak. No lectures. I ran the car at idle with the heat on high for more than two hours while continually gauging the coolant level. No issue with overheating. This too stopped the issue for about a week.
8. Both times I added stop leak, I ran the car pretty hard and replicated situations where it was overheating for an extended period of time without incident.

Eliminated issues:
1. I tried a new cap. Was not it.
2. The indicator is working. The car is running hot.
3. It's not a head gasket. Again, I've only lost coolant if it was actually running hot and through the cap. I'm not losing coolant.
4. Again, it is not a head gasket. There is no oil in the coolant or vice versa.
5. It can't be the thermostat stuck closed, as it would overheat on the highway.
6. The thermostat can't be stuck open, as it would run cool.

Possible but unlikely items:
1. A bad water pump could be the issue. But, it makes no sense that stop leak temporarily stopped and slowed the overheating under the same conditions.
2. For the same reason, I don't see the coolant temperature sensor being the culprit. Stop leak would have done nothing to affect the issue.
3. While I could try a different cap in the assumption I got a defective one, it would not explain the temporary stop leak fix. It's a closed system with the cap on the reservoir.
4. I don't think it's trapped air, as it's self bleeding and was an issue before I replaced the coolant.

Only possible thing I See:
1. Could it be a leak in the upper reservoir tank? It's a closed system. It has a two-piece plastic reservoir that is factory fitted together. The seam is just above the fill line. There are no visible cracks.